Human cytomegalovirus (HCMV) is a genus of viruses in the order Herpesvirales, in the family Herpesviridae, in the subfamily Betaherpesvirinae. There are currently eight species in this genus, which have been identified and classified for different mammals, including humans, monkeys, and rodents. The most studied genus is human cytomegalovirus, also known as human herpesvirus 5 (HHV-5), which is widely distributed in the human population. Diseases associated with HHV-5 include mononucleosis and pneumonias. All herpesviruses share a characteristic ability to remain latent within the body over long periods of time. Although they may be found throughout the body, CMV infections are frequently associated with the salivary glands in humans and other mammals. Other CMV viruses are found in several mammal species, but species isolated from animals differ from HCMV in terms of genomic structure, and have not been reported to cause human disease.
HCMV is endemic in most parts of the world. It is a ubiquitous large enveloped virus that infects 50 to 100% of the adult population worldwide. Although generally asymptomatic in immunocompetent hosts, HCMV infection is a major cause of morbidity and mortality in immunocompromised persons, such as infants following congenital or neonatal infections, transplant recipients, or AIDS patients.
Primary infection normally results in subclinical disease after which the virus becomes latent, retaining the capacity to reactivate at a later time. The virus is transmitted through body fluids, such as blood, saliva, urine, semen and breast milk. In particular, individuals with undeveloped or compromised immunity are highly sensitive to infection by HCMV. It is estimated that at least 60% of the US population has been exposed to CMV, with a prevalence of more than 90% in high-risk groups (e.g., unborn babies whose mothers become infected with CMV during the pregnancy or people with HIV).
In healthy individuals, HCMV typically causes an asymptomatic infection or produces mild, flulike symptoms. However, among two populations, HCMV is responsible for serious medical conditions. First, HCMV is a major cause of congenital defects in newborns infected in utero. Among congenitally infected newborns, 5-10% have major clinical symptoms at birth, such as microcephaly, intracranial calcifications, and hepatitis, as well as cytomegalic inclusion disease, which affects many tissues and organs including the central nervous system, liver, and retina and can lead to multi-organ failure and death. Other infants may be asymptomatic at birth, but later develop hearing loss or central nervous system abnormalities causing, in particular, poor intellectual performance and mental retardation. These pathologies are due in part to the ability of HCMV to enter and replicate in diverse cell types including epithelial cells, endothelial cells, smooth muscle cells, fibroblasts, neurons, and monocytes/macrophages.
The second population at risk are immunocompromised patients, such as those suffering from HIV infection and those undergoing transplantations. In this situation, the virus becomes an opportunistic pathogen and causes severe disease with high morbidity and mortality. The clinical disease causes a variety of symptoms including fever, pneumonia, hepatitis, encephalitis, myelitis, colitis, uveitis, retinitis, and neuropathy. Rarer manifestations of HCMV infections in immunocompetent individuals include Guillain-Barré syndrome, meningoencephalitis, pericarditis, myocarditis, thrombocytopenia, and hemolytic anemia. Moreover, HCMV infection increases the risk of organ graft loss through transplant vascular sclerosis and restenosis, and may increase atherosclerosis in transplant patients as well as in the general population. It is estimated that HCMV infection causes clinical disease in 75% of patients in the first year after transplantation.
There is currently no approved HCMV vaccine. Two candidate vaccines, Towne and gB/MF59, have completed phase II efficacy trials. The Towne vaccine appears protective against both infection and disease caused by challenge with pathogenic Toledo strain and also appears to be effective in preventing severe post-transplantation CMV disease. However, in a small phase II clinical trial, a low dose of Towne vaccine failed to show protection against infection of seronegative mothers who had children actively shedding CMV.
The gB/MF59 vaccine is a protein subunit vaccine comprised of a transmembrane-deleted version of HCMV gB protein, which induces high levels of fibroblast entry neutralizing antibodies in humans and has been shown to be safe and well tolerated in both adults and toddlers. A recent phase II double-blind placebo-controlled trial of the gB/MF59 vaccine revealed a 50% efficacy in inducing sterilizing immunity. As this vaccine induces potent antibody responses but very weak T-cell responses, the partial efficacy provided by the vaccine is thought to be primarily antibody-mediated. While this HCMV vaccine is the first to show any protective efficacy, its 50% protection falls short of the 80-90% desired for most vaccines.
In addition, antibody therapy has been used to control HCMV infection in immunocompromised individuals and to reduce the pathological consequences of maternal-fetal transmission, although such therapy is usually not sufficient to eradicate the virus. HCMV immunoglobulins (Igs) have been administered to transplant patients in association with immunosuppressive treatments for prophylaxis of HCMV disease with mixed results. Antibody therapy has also been used to control congenital infection and prevent disease in newborns. However, these products are plasma derivatives with relatively low potency and have to be administered by intravenous infusion at very high doses in order to deliver sufficient amounts of neutralizing antibodies.
HCMV is the leading viral cause of neurodevelopmental abnormality and other birth defects in children and the costs to society are substantial. Although antiviral therapy is available, the treatment with antiviral agents is imperfect and development of a CMV vaccine is the most promising strategy for preventing CMV infection. Given that the health and economic benefits of effective HCMV vaccines are significant, the US Institute of Medicine and US National Vaccine Program Office has categorized development of a CMV vaccine as a highest priority, but no candidate vaccine is under consideration for licensure.